The loss of teeth in the basal oviraptorosaurs is oftern overshadowed by their more famous (and larger) relatives, the Caenagnathidae and the Oviraptoridae. The former are largely grouped under a complex of bizarre long, broad, and fused mandibles (fusion in the mandible is particular only to them among oviraptorosaurs, or in fact among dinosaurs in general — excluding birds); the latter are noted for having very, very short and deep jaws, short and deep snouts, and distinctive features of the palate.

One of the characteristic synapomorphies for Oviraptorosauria in fact relates to the exposure of the three primary palatal bones below the oral margin (premaxilla, maxilla, jugal, and quadratojugal), as ordinarily only the pterygoid and at times a portion of the ectopterygoid are exposed. But more curious is the placement and evolution of the taxa that lie outside the Caenagnathidae and Oviraptoridae. These are the “Toothed Oviraptorosaurs,” even though some of them do not appear to possess teeth in all portions of their jaws. Teeth are preserved in the premaxilla and progressively lost posteriorly in the maxilla, while in the mandible the symphyseal region is devoid of teeth, but they are preserved more extensively posteriorly.

Oviraptorosaurian theropods begin as gracile, long-legged and short-armed dinosaurs, appearing as nothing so much as a stubby-winged secretary bird.

Skeletons of Caudipteryx. Top left, Caudipteryx zoui, based on NGMC 97-4 and 97-9; top left, Caudipteryx dongi, based on IVPP V12344; bottom, Caudipteryx sp. based on IVPP V12430. Scale bar represents ten centimeters. Skeletons are all to scale.

Cranial anatomy is relatively underdecribed. Each of these species are preserved from crushed, disarticulated skulls. Several features of the type and paratype skulls of Caudipteryx zoui [1] are undescriubable because they are rotated, partially buried in matrix, or upside down, such that various anatomical features, while potentially available (such as the braincase) are simply undescribable. Incisivosaurus gauthieri‘s skull [2], while preserved in such detail as to permit CT scans of its endocranium [3], is crushed mediolaterally, so that its aspect in all views is obscured; it is likely that the preorbital region of the skull and possibly the mandible are the least distorted regions of the skull.

Skeleton of Protarchaeopteryx robusta, based on the holotype NGMC 2127.

The skull of Protarchaeopteryx robusta [1,4] is so poorly preserved that only the broken premaxilla, portions of the preorbital cranium, and the mandible are exposed. The morphology nonetheless prompted [5] to synonymize Incisivosaurus into Protarchaeopteryx as P. gauthieri, on the basis of the deep premaxilla and the enlarged roots of the first pairs of premaxillary teeth, as well as the tiny marginal dentition on the maxilla and dentary.

Abberantly, a skull of the caudipterid-like oviraptorosaur Avimimus portentosus [6] (which also exhibits mediolateral crushing, is nonetheless highly modified with extreme shortening of the facial skeleton as indicated by the length of the infratemporal bar, posterior position of the postorbital, rostrocaudal shortening of the infratemporal fenestra, while at the same time showing extreme elongation of the pterygoid as in other oviraptorosaurs [5]. Various cranial reconstructions have been produced, but much of them have dealt largely with the argument that the skull obeyed typical theropod organization [7]; Kurzanov, as it should have been, produced the first [8], but deviated from the traditional model by proposing that Avimimus, as a bird, had an external naris that extended to the level of the orbit. This is supported by preservation of the partial nasal, which has a rostral margin preserved in a U-shape, and is seemingly unbroken, implying it forms a portion of a natural aperture, and in birds or nonavian theropods, the only likely aperture is the external naris. Even more curious is the preservation of a tiny fragment of what Kurzanov [6,8] identified as the premaxillae; these are paired triangular bones with a crenellated ventral margin, a lateral fossa, and are apparently part of the anterior rostrum; it is unlikely that they are maxillae or dentaries. Such a fossa should imply they extend the external naris nearly the length of the skull, and this has interesting implications for the skull, if ture. I have chosen to reconstruct the skull in a more typically theropod manner, although the dorsal and retracted position for the naris is not unlikely. Finally, a tiny L-shaped bone may belong to the dentary as a portion of the symphyseal section of the mandible; it also appears to be lightly crenellated [6,8].

Yet more interesting is that a skull cap forming a part of the holotype of Avimimus portentosus (PIN 3907/1) [8,9] is lower, longer, and less fused than the referred skull (PIN 3907/3); if the frontoparietal is correctly identified, even if it is dorsoventrally crushed, it suggests two different cranial morpholgies referred to Avimimus, and that PIN 3907/1 (shown among the skull above) may belong to a different taxon.

Similicaudipteryx yixianensis was described from a skeleton lacking a skull [10], preventing comparison; however, new specimens referred from the Jiufotang Formation (younger than the original formation) [11] preserve skulls, and these appear to be very similar to Caudipteryx sp.

With Incisivosaurus placed at the base of the oviraptorosaur tree [2], followed by Caudipteryx-like theropods, then by the split with Caenagnathidae and Oviraptoridae, it is inferred that loss of teeth was progressive, and moreover that it occured back to front in the upper jaw; its transition in the lower jaw in unknown, although there are clues.

As shown, the mandible of Microvenator celer [12,13], although not identified as such by Ostrom and questionably so by Makovicky and Sues, possesses features of the lingual surface such as pits and divots, which resemble the pitting of the mandible in Caenagnathasia martinsoni [14], with the additional quality in the latter of possessing a dentigerous shelf, as seen in pleurodont implantation. Mandibular dentition is present only in Incisivosaurus and Protarchaeopteryx [1,2,5], but in Caudipteryx zoui [1] and Caudipteryx sp. [15], the lingual surface resembles Microvenator in bearing slight pitting. This texture is absent in oviraptorids and there is a broad shelf with socket-like pits in caenagnathids [14]. If one were to place these dentaries on a transitional scale, then Caudipteryx would be closer to oviraptorids than even caenagnathids, and Caenagnathasia would be the sister taxon to the Caenagnathidae (Caudipteridae+Oviraptoridae) clade. This is not particularly borne out through non cranial anatomy [2,5], however those analyses differ, and one is left with the potential argument that either oviraptorosaurs lost their dentary dentition independantly in several lineages (caudipterids, caenagnathids, and oviraptorids) or that after loss, caenagnathids developed a lingual apparatus of the dentary.

Progressive loss of dentition seems more difficult than it appears on its face, and bears additional examination. At the same time, the anatomy of the jaw joint and palate and less well understood, especially in light of potentially convergent development of a convex mandibular joint on the articular [13,14].

This allows us to begin to dovetail into the caenagnathids-proper, which will here exclude Caenagnathasia martinsoni. It is my understanding that that taxon is unlikely to be a caenagnathid as argued by Currie at al. [14], but represents a more Microvenator/Caudipteryx level of oviraptorosaurian evolution.